This invention relates to the field of coding devices. More particularly, this invention relates to a coding device which is secure against unauthorized attempts to operate the device by human intervention.
Code operated devices are used for a variety of applications. One important application for code operated devices is for SAFE and ARM mechanisms for various kinds of missiles and other weapons. Such devices involve the use of codes to prevent arming of a weapon system unless a predetermined code has been satisfied. Coded SAFE and ARMING devices are particularly important for use with nuclear weapons, although their utility is by no means limited to such weapons.
A prior art electromagnetic code operated device is described in U.S. Pat. No. 4,099,161 granted to the present inventor and assigned to the assignee hereof (the subject matter of which is incorporated herein by reference). The device of that prior patent is an effective code operated device for a weapon or other system. In the environment of a weapon system, it operates to effect a change from a SAFE state to an ARMED state in response to receipt of a predetermined code in an appropriate signal input sequence. In the event an improper code is received, the device of that prior patent will lock to a DUD state, and the device will not advance further toward the ARMED state.
While effective for its intended purposes, the feature of the system of the U.S. Pat. No. 4,099,161 wherein it locks to a DUD state in response to receipt of an improper code can, if sufficient time is available, be used by a knowledgeable individual to work through the code to unlock and gain unauthorized access to the system. The system of the prior patent operates on a code of binary "0" and "1" inputs. If the proper code is delivered, the device will "unlock" to permit operation of the protected mechanism (missile, weapon, etc.); but the code wheel locks if an improper code sequence is delivered. Thus, if a "1" is inputted at the place in the code where an "0" is the proper input (or vice versa), the device will lock to the DUD state. However, while this locking prevents any further advance to the unlocked state, it will tell a knowledgeable person that the wrong binary signal was used at the place in the input signal string. Thus, a knowledgeable person will know that there is always a 50-50 chance of correctly guessing each succeeding bit in a code; and if the system locks because of an incorrect input, that person will know that the lock state can be avoided at this place in the input signal string by using the complementary binary input at that place. Thus, it may be possible to work through the code and unlock the device by
(1) inputting a string of binary signals until the device locks, PA0 (2) resetting the device to the start position and repeating the previous string of input signals with the last one (i.e. the one that caused the system to lock) being changed to the opposite binary state, PA0 (3) continuing to input signals until another lock state is encountered, and PA0 (4) repeating steps (2) and (3) until the enter code is worked through and the device unlocks.
Thus, while the device of U.S. Pat. No. 4,099,161 is a coded safety device, it is not considered a secure device in the sense that it is protected against reasoned human intervention. With a binary code it is, of course, statistically possible that someone could randomly guess the correct code. While the probabilities of that happening are very low, there does not appear to be any way to make a device secure against that remote possibility. However, it is highly desirable to make coded safety devices secure in the sense of protecting them against attempts at reasoned human intervention.